1. Field of the Invention
This invention is related to ranging apparatus and more particularly relates to apparatus for encoding the ranging signal to prevent decoding of the signal, thus, preventing spoofing and making detection of the source difficult.
2. Description of the Prior Art
Radio frequency altimeters are well known. Most commercial aircraft altimeters employ either pulse modulation or frequency modulation techniques. Both techniques are not covert and are easily detectable, thus, are not suited for military aircraft and missiles. Both systems send out a recognizable feature in the transmitted signal and measure the time required for the return of the recognizable feature. The measured time for the round trip of the signal is an analog of the range.
It has been suggested that a pseudo-noise encoded waveform may be placed on a modulated carrier and transmitted to make the ranging source both difficult to detect and to decode. The recognizable feature is ordinarily recognizable to the source equipment and is usually indistinguishable from the low power spectral density waveform.
Pseudo-noise continuous wave altimeters are possible to intercept, and using sophisticated computer equipment, can be decoded so as to employ electronic countermeasure spoofing equipment and make the range measurements incorrect.
There is a need for an accurate radio frequency altimeter in which the recognizable feature in the encoded waveform has a remote statistical probability of being decipherable and the source of the encoded waveform signals is difficult to detect.